The present invention relates to wireless power transfer systems, and more particularly to such systems incorporating circuitry and/or methodologies for attenuating or cancelling undesirable spurious signals, such as harmonics.
A wireless power transfer system 10 for transferring power from a charger 12 to a portable device 14 is shown in FIG. 1. There is a DC power source 16 in the charger 12 which is converted to an alternating current using an inverter 18. Typically a half-bridge circuit is used to generate a rectangular signal of either high voltage and/or high current. This signal is applied to a primary inductive coil 20. Within the portable device 14 there is a secondary coil 22 which is electromagnetically coupled to the primary coil 20 when the device is appropriately positioned on the charger 12. An alternating voltage is induced within the secondary coil 22. This voltage may be rectified using a diode bridge 24 to provide a DC voltage. This DC voltage may require some subsequent DC/DC conversion (not shown) to provide a voltage suitable for recharging the internal batteries (not shown) and powering the device 14.
In order to make the system 10 easier to drive, it is common to use resonance on the primary 20 and/or the secondary 22. Capacitors 26 and/or 28 may therefore be positioned on the primary and secondary side respectively, such that both the primary side and secondary side are resonant at the frequency of the inverter 18. An additional feature of this resonance is that it acts as a band-pass filter, thereby attenuating the unwanted harmonic components present in the rectangular signal.
In a wireless charger, it is inevitable that any frequency components present on the primary coil will be radiated into the atmosphere, because it is not possible to adequately shield the coil and still permit coupling to a portable device. It is therefore necessary to select a fundamental frequency in a band where emissions are permitted by legislation. However the primary inductor can act as a very efficient antenna at higher harmonics and this can cause unwanted interference. This is true even when the harmonics are attenuated by the resonant system, because the fundamental has to be high enough to supply sufficient power to the portable device.
A further issue which can exacerbate the situation is that a rectifier is required in the portable device. This is typically a nonlinear component which is directly coupled to the primary coil. Harmonics are generated by the rectifier and coupled back into the primary coil where they are radiated.